Re: Performance measurements (thanks and conclusion)

[Martin Kahlert <martin.kahlert@mchp.siemens.de>]

Quoting Robert Wilhelm (robert@physiol.med.tu-muenchen.de):
> > [Robert: could you please compile my code on your Alpha using
> >          egcs and report your results to me]
> 
> I get about 275 MFLOPS for my 533MHz 21164a for both egcs 1.0 and
> egcs-current with haifa enabled.
> 
> If I use different local variables lfA*, egcs seems to shedule a bit
> better and I get 290 MFLOPS.
> 
> Robert

I was really overwhelmed with the repsonse to this thread on 
axp-list. Thanks a lot for all people who tried my source
and even tried to get more out of the compilers.

I tried both versions on my PPro 200:
Stefan Schroepfer's version:
pgcc:
85.98 MFLOPS
gcc-2.7.2.1:
97.10 MFLOPS
gcc-without double align:
95.46 MFLOPS
egcs-2.91.42:
84.06 MFLOPS
tcc:
17.20 MFLOPS

Robert Wilhelm's version:
pgcc:
81.62 MFLOPS
gcc-2.7.2.1:
98.81 MFLOPS
gcc-without double align:
98.81 MFLOPS
egcs-2.91.42:
83.44 MFLOPS
tcc:
16.44 MFLOPS

It seems that tcc is not the fastest and the most reliable 
under the sun...

Can i conclude, that it's a good idea to insert as many local
vars as possible to get good results from compilers?

Now i have two questions:

-Why is it so difficult for gcc to transform the code
 for(i=0;i<n;i++)
    result[i]=a[i]+2*a[i+1]+3*a[i+2];

 into something like

 _tmp0=a[0];_tmp1=a[1];_tmp2=a[2];
 for(i=0;i<n;i++)
    {
     result[i]=_tmp0+2*_tmp1+3*_tmp2;
     _tmp0=_tmp1;
     _tmp1=_tmp2;
     _tmp2=a[i+2];
    }
 for itself? I think, especially in Fortran such things are a
 common task.
-What's the reason for the performace loss between gcc-2.7.2.1 
 and egcs-2.91.42 - it's nearly 20%, that gcc-2.7.2.1 is better?

Thanks a lot,
Martin.